Triathlon/time trial bicycles are road bikes which are used in a time trial rather than a road race. These races are against the clock and the riders are set off at set intervals. These are very often one minute, but sometimes two minutes in professional events and team time trials.
Time trial riders (also known as testers) are not allowed to draft behind other riders (ride in their slipstream). A time trial bike is therefore designed to be very aerodynamic (aero) helping the rider reduce the aero drag of the bicycle and the rider.
Time Trial bikes are fitted with tri-bars (or aero-bars) rather than the more common drop handlebars usually fitted to road bikes. These tri-bars are designed to give the rider good stability and an aero riding position where the forearms rest on pads with hands gripping the extended bars. The gear levers are often fitted to the bars enabling the rider to change gear while staying in the aero position.
These aero-bars became popular following the Tour de France of 1989 when Greg Lemond made up 50 seconds on the final time trial stage, beating Laurent Fignon (who was the race leader at the time).
Most Time Trial races are a set distance; usually 10 miles, 25 miles or 50 miles (with some longer distance and time endurance events). The short time spent on the Time Trial bike means the design does not need to concentrate as much on comfort or handling as the courses tend not to be as technical as a road race.
The main differences between a road bike and a Time Trial bike are as follows:
1)Since time trials are usually flat, the bikes tend to be designed for aerodynamics rather than weight and handling.
2)They tend to have higher gearing due to the nature of course and duration of the event.
3)The wheels are usually deep rim or a solid disk designed to help with the aero drag.
4)Some frames are designed with shaped tubes, again to help with aerodynamics.